The present invention relates generally to filtering a signal read from a data storage medium and, more particularly, to a system and method for calibrating a cutoff frequency of a tunable filter used for filtering a signal read from a data storage medium.
Tunable filters, such as integrated continuous time filters (CTFs), are known to be particularly useful when implemented in the read channel of a disk drive system for purposes of filtering readback signals obtained from a data storage disk. A typical disk drive system includes a magnetic medium for storing data in magnetic form and a number of transducers used to write and read magnetic data respectively to and from the medium. Digital information is typically stored in the form of magnetic transitions on a series of concentric, spaced tracks formatted on the surface of the magnetizable rigid data storage disks.
Reading data from a specified disk location is typically accomplished by using a read element of the read/write head assembly to sense the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals, commonly referred to as readback signals, in the read element.
A filter employed in a read channel must generally be capable of accommodating variations in the data rate of the readback signals transmitted through the read channel. The ability to precisely tune such a filter is therefore of particular importance. It is generally considered imperative that a cutoff or corner frequency of a tunable filter used in such applications be adjustable with a high degree of precision. Such applications may also require that the tunable filter be adjustable within a range of selectable cutoff frequencies with high precision.
It is well understood in the art that tunable filters implemented in integrated circuits, such as VLSI chips for example, exhibit frequency response characteristics that are sensitive to technology and process variations, Technology variations, as well as factors resulting from circuit/filter design or implementation, often result in large tolerances with respect to the filter""s specified cutoff frequency. Such tolerances can result in performance loss.
There exists a keenly felt need for an apparatus and method for precisely adjusting the cutoff frequency of a tunable filter to a specified frequency or frequencies. There exists a need in the disk drive system manufacturing community for such an apparatus and method suitable for implementation in-situ a disk drive system and, more particularly, in-situ a read channel of the disk drive system. The present invention fulfills these and other needs.
The present invention is directed to an apparatus and method for calibrating a cutoff frequency of a tunable filter, such as a tunable filter coupled to a read channel of a data storage system. A calibration circuit according to the present invention includes a variable gain amplifier (VGA), an envelope detector, and a tunable filter coupled between the amplifier and the envelope detector. A calibration signal having a preestablished frequency representing a specified cutoff frequency associated with the tunable filter is generated and applied to the VGA. The gain of the amplifier is adjusted until an amplitude of an output signal provided by the filter is substantially equal to a first predetermined amplitude.
A response of the filter is then adjusted until an amplitude of an output signal provided by the filter is substantially equal to a second predetermined amplitude, at which time the desired cutoff frequency of the tunable filter is precisely established. A cutoff frequency calibration procedure according to the present invention may be implemented for calibrating a single cutoff frequency or, in addition, may be implemented for calibrating the low and high cutoff frequencies of a range of cutoff frequencies associated with a tunable filter.
Adjusting the gain of the amplifier involves applying voltage control signals to the amplifier. In one embodiment, a control circuit produces digital words representative of voltage control signals which are applied to the amplifier via an N-bit gain digital-to-analog converter (DAC) and a gain control buffer or multiplexer. Adjusting the response of the filter involves adjusting the cutoff frequency of the filter, preferably while the gain of the VGA is held at the previously adjusted level. In one embodiment, the response of the filter is adjusted by applying digital calibration bits to the filter.
A circuit for calibrating a cutoff frequency of a tunable filter includes a variable gain amplifier coupled to the filter and to a calibration signal source. The filter, in one embodiment, comprises a tunable lowpass filter The calibration signal source generates a calibration signal having a preestablished frequency representative of a specified cutoff frequency of the filter. An envelope detector is coupled to the output of the filter. The envelope detector includes a number of analog comparators which are used to compare an output signal provided by the filter with each of a first reference voltage signal and a second reference voltage signal in response to a calibration signal applied to the amplifier.
A control circuit, coupled to the filter, amplifier, and envelope detector, controls the gain of the amplifier until an amplitude of an output signal provided by the filter is substantially equal to an amplitude of the first reference voltage signal. The control circuit then adjusts the cutoff frequency of the filter until an amplitude of an output signal provided by the filter is substantially equal to an amplitude of the second reference voltage signal.
In one embodiment, the control circuit is coupled to an N-bit gain DAC which, in turn, is coupled to a multiplexing circuit. The multiplexing circuit, which may be implemented as part of a gain control buffer, is coupled to the amplifier. The filter comprises a cutoff adjust input, a low cutoff frequency calibration input, and a high cutoff frequency calibration input, where each of the respective inputs is coupled to a respective output of the control circuit. The respective inputs of the filter are preferably digital inputs, and the respective outputs of the control circuit are preferably digital outputs. A cutoff frequency calibration circuit according to the present invention may be advantageously implemented in-situ a read channel of a disk drive system.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.
FIG. 1 is a top perspective view of a disk drive system with its upper housing cover removed;
FIG. 2 is a side plan view of a disk drive system comprising a plurality of data storage disks;
FIG. 3 is a diagram of circuitry typically provided in the read channel of a disk drive system, the circuitry including a tunable filter and circuit elements that provide for high precision adjustment of a cutoff frequency of the tunable filter in accordance with the principles of the present invention;
FIG. 4 is a graphical illustration of the frequency response of a tunable lowpass filter having a cutoff frequency that is adjustable according to an embodiment of the present invention;
FIG. 5 depicts in flow diagram form several steps involving the calibration of a cutoff frequency representing the low end of a range of cutoff frequencies associated with a tunable lowpass filter according to the present invention;
FIG. 6 depicts in flow diagram form several steps involving the calibration of a cutoff frequency representing the high end of a range of cutoff frequencies associated with a tunable lowpass filter according to the present invention;
FIG. 7 illustrates circuitry associated with an envelope detector provided in the read channel of a disk drive system for calibrating a cutoff frequency of a tunable filter coupled to the envelope detector in accordance with an embodiment of the present inventions;
FIGS. 8 and 9 are graphical representations of the frequency response of a tunable filter subject to a cutoff frequency calibration procedure in accordance with an embodiment of the present invention;
FIG. 10 depicts in flow diagram form several steps involving the calibration of a cutoff frequency representing the low end of a range of cutoff frequencies associated with a tunable lowpass filter in accordance with an embodiment of the present invention; and
FIG. 11 depicts in flow diagram form several steps involving the calibration of a cutoff frequency representing the high end of a range of cutoff frequencies associated with a tunable lowpass filter in accordance with an embodiment of the present invention.